This invention relates to a method for controlling the operation of a steel refining converter and, in particular, to a method for controlling a converter of the type wherein a combination of gases is blown into the melt through tuyeres located at the bottom of the converter.
In a conventional process for refining steel, oxygen is blown into a vessel through a lance positioned above the iron melt. While this process is satisfactory for many purposes the mixing of the batch is not complete enough for some applications, iron losses are relatively high and only a portion of the oxygen issuing from the lance is utilized. An improved process for refining steel employs oxygen blown from below the surface of the melt resulting in better mixing, higher efficiency and less smoke generation than the conventional method. The improved process may also include the use of side tuyeres mounted above the melt as an additional means of introducing oxygen.
A converter employed in carrying out this improved method comprises a tiltable vessel having a refractory lining and a bottom member provided with a plurality of nozzles, or tuyeres, extending through the bottom member. Each tuyere consists of a center jet through which oxygen flows during the refining portion of the process and an annulus jet concentrically surrounding the center jet through which a fuel gas flows to provide cooling for the center jet. Apparatus of this type is disclosed in copending U.S. Patent Application Ser. No. 800,892, filed Feb. 20, 1969, now U.S. Pat. No. 3,706,549 granted on Dec. 19, 1972.
Although oxygen is used in the center jet during the refining operation, various combinations of gases are required for purging, cooling the tuyeres and during other parts of the process such as charging the converter, sampling the resulting melt, tapping the converter after the iron has been refined and during the transition periods when the converter is being rotated to a position in which the next operation can take place. With the converter on its side during the charging, sampling and tapping operations, the tuyeres may be protected by the introduction of purging gases such as compressed air at the center jets and low pressure nitrogen at the annulus jets. When the vessel is being raised to its upright position for the refining operation, the pressure at the jets must be increased to assure that the molten metal will not enter the tuyeres thereby blocking the openings and allowing them to come into contact with the steel and highly corrosive slag. Nitrogen, at a relatively high pressure, may be substituted for the compressed air during this portion of the cycle.
After the converter is in its upright position and located under a hood which carries the gases away, the refining operation is carried out by substituting oxygen for the nitrogen at the center jet and a fuel for the nitrogen at the annulus jet. The pressure during refining must be high enough to prevent the nozzles from becoming blocked or damaged by contact with the melt. When the refining step has been completed, high pressure nitrogen is substituted for the oxygen and the converter tilted downward to permit drawing a sample or removing the completed charge. During the sampling or tapping operations, compressed air or low pressure nitrogen is substituted for the high pressure nitrogen at the center jets in order to prevent contamination of the surrounding area since the mouth of the converter is no longer under the hood.
From this brief description of the bottom-blown process for producing steel, it will be clear that adequate pressure and gas flow must be preserved at the tuyeres whenever they are covered by molten metal so that the metal does not enter the nozzles or connecting gas lines. If this should occur, severe damage would result to the equipment and the resulting conditions might be hazardous to personnel. Accordingly, a control system is highly desirable which will prevent the vessel from being tilted upright unless proper pressure has been provided on the tuyeres and which will assure that adequate pressure and gas flow is maintained on the tuyeres at all times. Further, there must be a smooth transition from one gas to another whenever a change is being made. Such a control system is provided by the present invention.